DSM-G600, DNS-3xx and NSA-220 Hack Forum

I've been seeing abysmally poor performance from my Macs connecting to a DNS-323 via my 802.11n Airport Extremes, while other devices on the same network have been working without issue. Some quick Googling for 'OS-X samba performance' lead me to try tweaking a few TCP stack parameters, with one change resulting in a dramatic performance improvement: setting delayed_ack to zero.

To test it yourself, start up a large transfer from your DNS-323 that would normally take a considerable amount of time. If you have a bandwidth monitoring tool, use it to measure performance objectively (I like iStat Menus for basic traffic monitoring). While the transfer is in progress, type the following:

sudo sysctl -a | grep net.inet.tcp.delayed_ack

You will see that the default value is set to 3. Now change it temporarily (will reset on reboot or if you set it back manually) to zero:

sudo sysctl -w net.inet.tcp.delayed_ack=0

If this affects you, you'll see an immediate and dramatic jump in the speed of your transfers. Mine went from fluctuating between ~60KB/s and 300KB/s to between 4MB/s and 5MB/s (yes, that's a capital 'B'.) You can switch back to the original setting by specifying a 3 instead of a 0:

sudo sysctl -w net.inet.tcp.delayed_ack=3

Switch back and forth to verify that the change does affect you. If so, you'll probably want to make the change permanent.

Add the following line to /etc/sysctl.conf:

net.inet.tcp.delayed_ack=0

Let me know if this helps anyone.

-Eric

P.S. Blogged about it here: DNS-323 Mac OS X Performance

Last edited by ericlitman (2007-10-10 01:48:57)